The present invention relates to a catalyst for purifying an exhaust gas, a process for producing the same, and a method using the catalyst for purifying an exhaust gas, particularly, to a catalyst which can efficiently purify nitrogen oxides (NOx) in an exhaust gas which contains oxygen excessively in an amount more than necessary for oxidizing carbon monoxide (CO) and hydrocarbons (HC) which are contained in the exhaust gas, a process for producing the same and a method for purifying the exhaust gas.
Conventionally, as a catalyst for purifying an automobile exhaust gas, a 3-way catalyst has been employed which carries out the oxidation of CO and HC and the reduction of NOx simultaneously to purify an exhaust gas. With regard to such a catalyst, for example, a catalyst has been known widely in which a loading layer comprising xcex3-alumina is formed on a heat-resistant support, such as cordierite, and a noble metal, such as Pt, Pd and Rh, is loaded on the loading layer.
By the way, the purifying performance of such a catalyst for purifying an exhaust gas depends greatly on the air-fuel ratio (A/F) of an engine. For example, when the air-fuel ratio is large, namely on a lean side where the fuel concentration is lean, the oxygen amount in the exhaust gas increases so that the oxidation reactions of purifying CO and HC are active, on the other hand, the reduction reactions of purifying NOx are inactive. Conversely, for example, when the air-fuel ratio is small, namely on a rich side where the fuel concentration is high, the oxygen amount in the exhaust gas decreases so that the oxidation reactions are inactive and the reduction reactions are active.
Whilst, in automobile driving, in the case of urban driving, the acceleration and deceleration are carried out frequently so that the air-fuel ratio varies frequently within the range of from adjacent to the stoichiometric point (ideal air-fuel ratio) to the rich state. In order to cope with the low fuel consumption requirement in such driving, a lean-side driving is needed in which a mixture containing oxygen as excessive as possible is supplied. Therefore, it is desired to develop a catalyst which can fully purify NOx on the lean side as well.
Hence, an NOx-storage and reduction type catalyst has been proposed in which an alkaline-earth metal and Pt are loaded on a porous support, such as alumina (Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, etc.). In accordance with this catalyst, since the NOx are absorbed in the alkaline-earth metal, serving as the NOx storage material, and since they are reacted with a reducing gas, such as HC, and are purified, it is good in the purifying performance of NOx even on the lean side.
In the catalyst disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, it is believed that barium, for example, is loaded as the carbonate, and the like, on the support, and it reacts with NOx to generate barium nitrate (Ba(NO3)2), thereby storing the NOx.
That is, in the above-described NOx-storage and reduction type catalyst, by controlling the air-fuel ratio from the lean side to the stoichiometric point and to the rich side in a pulsating manner, the NOx are stored in the NOx storage material on the lean side. And, the stored NOx are released at the stoichiometric point and on the rich side, are reacted with the reducing components, such as HC and CO, by the catalytic action of Pt, and are thereby purified. Therefore, since the emission of the NOx is inhibited even on the lean side, a high NOx purifying ability is exhibited as a whole.
In addition, it is understood that the purifying reaction of the NOx in the NOx-storage and reduction type catalyst comprises a first step of oxidizing NO in an exhaust gas to NOx, a second step of storing the NOx on the NOx storage material, and a third step of reducing NOx, which are emitted from the NOx storage material, on the catalyst.
However, in the exhaust gas, SO2 is contained which is generated by burning sulfur (S) contained in the fuel, it is further oxidized to SOx, such as SO3, by the catalytic metal in an oxygen-rich atmosphere. Then, they are easily turned into sulfuric acid by the water vapor contained in the exhaust gas, and they are reacted with the barium, etc., to generate sulfites and sulfates, and it is understood that the NOx storage material is thus poisoned and degraded. This phenomenon is referred to as sulfur poisoning. Moreover, the porous support, such as alumina, has a property that it is likely to absorb the SOx, and there is a problem in that the aforementioned sulfur poisoning is facilitated.
And, when the NOx storage material is turned into the sulfites and the sulfates, it cannot store the NOx any more, and, as a result, there is a drawback in the aforementioned catalyst in that the NOx purifying ability decreases gradually in the service.
Moreover, since titania (TiO2) does not absorb SO2 it was thought of using a TiO2 support, and an experiment was carried out. As a result, SO2 was not absorbed by the TiO2 and flowed downstream as it was, since only the SO2, which contacted directly with the catalytic noble metal, was oxidized, it was revealed that the sulfur poisoning occurred to a lesser extent. However, when the TiO2 support is employed, the initial activity was low, and it was revealed that there was a critical drawback in that the NOx purifying performance was kept low after durability.
Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 6-327,945, it is proposed to use a support in which alumina is mixed with a composite oxide, such as a Baxe2x80x94Ce composite oxide and a Baxe2x80x94Cexe2x80x94Nb composite oxide. In addition, in Japanese Unexamined Patent Publication (KOKAI) No. 8-99,034, it is proposed to use at least one composite support selected from the group consisting of TiO2-Al2O3, ZrO2xe2x80x94Al2O3 and SiO2xe2x80x94Al2O3. By thus using the support in which the composite oxide is mixed, or by using the composite support, the NOx storage material is inhibited from the sulfur poisoning, and the NOx purifying ability after durability is improved.
However, since the recent increase of the high-speed driving, the improvement in the engine performance, and the regulation of the exhaust gas accompany the highly elevated exhaust-gas temperature, the exhaust-gas purifying catalyst is required to exhibit a further enhanced heat resistance.
The present invention has been developed in view of the aforementioned circumstances, and it is a primary object of the present invention to enable the NOx storage material to be further inhibited from the sulfur poisoning, and to be capable of keeping a high NOx conversion ratio even after the service at an elevated temperature.
A characteristic of a catalyst for purifying an exhaust gas according to the present invention, solving the aforementioned assignments, is that the catalyst, which is of an NOx storage-and-reduction type, and disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more, so that NOx in the exhaust gas is stored therein, and whose air-fuel ratio is perturbed from the stoichiometric point to a fuel-rich atmosphere periodically so that the NOx stored therein is released therefrom, thereby carrying out reducing and purifying, comprises: a support including rutile type titania; an NOx storage material including at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support; and a noble metal loaded on the support.
A characteristic of a process for manufacturing a catalyst for purifying an exhaust gas according to the present invention is that the process comprises the steps of: heat-treating by contacting a rutile type titania source with an NOx storage material source including at least one element selected from the group consisting of alkali metals, alkaline-earth metal and rare-earth elements and by heat-treating them at 500-1,000xc2x0 C., thereby forming a composite oxide powder of rutile type titania and an NOx storage material; mixing the composite oxide powder and an alumina powder, thereby making a support powder; and loading a noble metal on the support powder.
Moreover, a characteristic of a method for purifying an exhaust gas according to the present invention is that a catalyst, comprising a support including rutile type titania, an NOx storage material including at least one element selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements and loaded on the support, and a noble metal loaded on the support, is disposed in an exhaust gas of an oxygen-rich atmosphere made by burning an air-fuel mixture whose air-fuel ratio, A/F (air/fuel), is 18 or more so that NOx in the exhaust gas are stored in the NOx storage material, and whose air-fuel ratio is perturbed from the stoichiometric point to a fuel-rich atmosphere periodically so that the NOx stored in the NOx storage material are released, thereby carrying out reducing and purifying.